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Memoir 16— Stratigraphy of the Big Hatchet Mountains area, New Mexico

By R. A. Zeller, Jr., 1965, reprinted 1985, 128 pp., 3 tables, 19 figs., 6 plates, 1 index. This stratigraphic study was concentrated in the Big Hatchet Mountains of southwestern New Mexico, where Paleozoic rocks are completely exposed, and in Mojado Pass to the south, where Cretaceous rocks crop out. Prior to the present study, only reconnaissance observations had been made of the stratigraphy of the Big Hatchet Mountains area.

A remarkably thick and complete sequence of sedimentary formations lies between Precambrian basement rocks and Tertiary volcanic rocks. The column of sedimentary rocks exposed is about 20,000 ft thick, half of which is Paleozoic and half of which is Lower Cretaceous. The lower 15,000 ft of rocks is almost entirely of marine origin; the upper 5,000 ft is mostly terrestrial.     Paleozoic rocks rest upon erosion surface cut on Precambrian granite and quartzite. Cambrian, Ordovician, and Devonian rocks represented by the Bliss, El Paso, Montoya, and Percha Formations are similar to the corresponding formations elsewhere in southern New Mexico. Silurian rocks are absent. Mississippian rocks, divided into the Escabrosa Limestone below and the Paradise Formation above, take their names from the formations of southeastern Arizona which they strongly resemble. Likewise, the Pennsylvanian and Permian sequence in the Big Hatchet Mountains area resembles the corresponding sequence in southeastern Arizona. Therefore, it is assigned to the Naco Group and is divided into the Arizona formations, which in ascending order, are Horquilla Limestone, Earp Formation, Colina Limestone, Epitaph Dolomite, Scherrer Formation, and Concha Limestone.

The Paleozoic section is truncated throughout the region by an erosion surface upon which rest Lower Cretaceous rocks. In the Big Hatchet Mountains, this erosion surface was cut to various depths into the Concha Limestone. Arkosic sediments of the basal Lower Cretaceous section were derived from granite that was exposed nearby because of orogeny during the Permian-Early Cretaceous hiatus.

The Lower Cretaceous section is divisible into three gross lithologic units or formations-red beds below, limestone in the middle, and sandstone above. Though these formations resemble certain others in the region in lithology and age, correlations can't be confidently established because correlations are not possible at present with similar-appearing formations only a few miles away in the Little Hatchet Mountains. Thus, the formations are given new names, which in ascending order are Hell-to-Finish Formation, U-Bar Formation, and Mojado Formation. Steeply dipping Cretaceous rocks are overlain unconformably by gently dipping Tertiary fanglomerate followed upward by a thick pile of volcanic rocks.

During most of Paleozoic and Early Cretaceous time, the Big Hatchet Mountains area was the site of greater deposition and thus greater subsidence than bordering areas in New Mexico and Arizona. Paleozoic formations from the Mississippian through Permian are thicker here than elsewhere, and Lower Cretaceous deposits are very thick. Such thicknesses suggest geosynclinal deposition. Fossil collections from most of the formations were submitted to a number of specialists whose identifications and comments on the ages of faunas and floras are given. Correlations of formations are suggested. Stratigraphic sections are described and shown graphically.

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